1. Field of the Invention
This invention relates to switching voltage regulators utilizing pulsewidth modulated control of the switching device. It is particularly concerned with a switching voltage regulator utilizing digital type average current control techniques.
In typical existing peak current control systems for control of switching voltage regulators, current passing through the power switching transistors is monitored on an instantaneous basis. As the current through the switching transistor increases its instantaneous magnitude is continuously compared with a particular current threshold reference value. When the current threshold value is reached, the switching transistor is biased nonconducting. The current threshold value establishing the turn-off time of the switching transistor is controlled by a voltage feedback circuit which operates to control the output voltage of the switching regulator.
Controlling the output voltage of a switching regulator or converter circuit by peak current control advantageously permits fast current limiting and eliminates an occurrence of possible damage to the switching transistor from short circuits occurring at the output of the regulator. A typical analog type peak current control system applied to a converter circuit is disclosed in U.S. Pat. No. 4,148,097 issued to C. W. Deisch on Apr. 3, 1979 and assigned to the same assignee as this application.
Peak current control as previously utilized has limitations. For example, by monitoring the instantaneous value of the switching transistor current, the regulation circuitry is rendered very susceptible to transient variations of this current, such as ringing at the initial turn-on of the switching transistor. In addition under certain operating conditions, the peak current control becomes unstable. These conditions are generally related to the slope and duration of current increase through the switch within a given switching period.